Impedance measurement in a high-voltage power system

ABSTRACT

A power monitoring instrument evaluates and displays the source impedance, load impedance, and distribution system impedance of an alternating current power system using voltage and current measurements taken at a source and load of the power system. The power monitoring instrument and associated voltage and current pods for coupling to voltage probes and current clamps incorporate unique safety features to minimize operator exposure to high-voltage. The measurements are performed noninvasively without disconnecting elements of the power system. Evaluation of impedances takes advantage of incidental variations in the load characteristics.

This is a Division of application Ser. No. 08/248,280 filed May 24, 1994now U.S. Pat. No. 5,514,969.

BACKGROUND OF THE INVENTION

This invention relates generally to an apparatus and method forconveniently measuring impedances in a high voltage alternating currentpower system while minimizing operator exposure to dangerous voltagelevels.

In troubleshooting a high voltage power system such as the power systemof a building, it is often useful to analyze impedance characteristics,whether of a power source, load, or intermediate wiring. For reliableperformance, the source impedance should be less than 1/20 thedistribution and load impedances. As the source impedance increasesrelative to the load impedance, the voltage delivered to the loaddecreases and voltage waveform distortion can develop.

Prior art impedance measurement techniques are cumbersome and invasive.For example, measuring the impedance of a source requires disconnectingthe source from its load and substituting an adjustable test load.Impedance is then derived from changes in source voltage resulting fromvarying currents drawn by the load.

Furthermore, performing the necessary voltage and current measurementsin a high-voltage environment poses safety problems. If the powermonitoring instrument is located adjacent to the points to be monitored,the operator is exposed to the danger of electrical shock for a longtime. If the voltage probes and current clamps coupled to the necessarymonitoring points are remote from the power monitoring instrument,shuttling between the instrument and probes is necessary to assure thatcontact has been made by the voltage probes and that the current clampsare secured in the desired orientation. Repeated visits to themonitoring points and handling of the probes and clamps increase therisk of shock. The problem is exacerbated when a multi-phase powersystem is to be monitored and probes or clamps may be accidentallycoupled to the wrong phase.

Danger is also presented by the need to handle high-voltage connectionsto differential inputs at the rear of the power monitoring instrument.When struggling under protective clothing in a hot equipment closet, orin cold weather, it is easy to short phases together or to neutral witha jumper cable.

What is needed is a non-invasive apparatus and method for determiningimpedances of a source and load in a high-voltage power system. Theapparatus and method should be convenient and should minimize operatorexposure to danger of shock when performing the necessary measurements.

SUMMARY OF THE INVENTION

In accordance with the invention, a power monitoring instrumentevaluates and displays the source impedance, load impedance, anddistribution system impedance of an alternating current power systemusing voltage and current measurements taken at a source and load of thepower system. The power monitoring instrument and associated voltage andcurrent pods for coupling to voltage probes and current clampsincorporate unique safety features to minimize operator exposure tohigh-voltage. The measurements are performed noninvasively withoutdisconnecting elements of the power system. Evaluation of impedancestakes advantage of incidental variations in the load characteristics.

The power monitoring instrument of the invention monitors loadimpedance, defined as the ratio of rms voltage to rms current, forsuccessive cycles of an alternating current signal generated by thepower source to identify a pair of cycles with disparate loadimpedances. The source impedance is determined from the ratio of thechange in rms voltage to the change in rms current between the cycles ofthe identified pair.

Successive estimates of source impedance are generated and stored forsuccessive identified pairs of cycles having disparate load impedances.Cumulative statistical data is maintained for the stored sourceimpedance estimates and the mean source impedance is displayed once thestored estimates show long-term consistency as evidenced by a lowstandard deviation.

A complete impedance diagram of a power system can be generated bydetermining source impedance at the utility service entrance or point ofcommon coupling in accordance with the invention and then repositioningprobes and clamps to measure voltage and current at the load. Loadimpedance is then evaluated to be the ratio of load voltage to loadcurrent. An apparent source impedance is evaluated at the load byapplying the techniques described to determine source impedance tomeasurements made at the load. The distribution system impedance canthen be derived by subtracting the source impedance as measured at thepoint of common coupling from the apparent source impedance as measuredat the load. The complete wiring diagram including source impedance,load impedance, and distribution system impedance is then generated anddisplayed, facilitating troubleshooting and analysis.

To minimize operator exposure to high voltage when physically attachingprobes to perform the necessary measurements, the power monitoringinstrument of the invention incorporates unique safety features. Avoltage pod is coupled to the power monitoring instrument by aconnection cable. The voltage pod connects via signal leads to aplurality of voltage probes. Resistive dividers are incorporated intothe pod so that high-voltage is not present at the instrument. LEDscorresponding to each voltage probe are incorporated into the pod. Anillumination of an LED indicates a live connection so the operator neednot shuttle between the instrument and probes to verify good contact.

In accordance with the invention, a current pod is coupled to the powermonitoring instrument by a lengthy connection cable. The current pod isconnected by signal cables to a plurality of current monitoring clamps.The current pod also incorporates an LED for each associated currentclamp and a steady illumination of the LED indicates good couplingbetween the clamp and a conductor to be monitored while a flashing ofthe LED indicates an overrange condition.

Circuitry within the power monitoring instrument allows an operator tocorrect for inadvertent misconnection phase conductors by the currentclamps or voltage probes by entering special commands. Similarly,corrections may also be made for misoriented current monitor clamps. Inaddition to enhancing safety, the ability to correct connections withouthandling probes and clamps provides convenience in that shuttlingbetween the power monitoring instrument and monitoring points isreduced.

The invention will be better understood upon reference to the followingdetailed description in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a power monitoring instrument and associated voltage podin accordance with the invention;

FIG. 2 depicts a representation of a voltage pod interface in accordancewith the invention;

FIG. 3 is a flowchart describing the steps of indicating the presence ofa signal on a given voltage probe in accordance with the invention;

Fig, 4 depicts a phasor diagram display wherein voltage probes and phaseconductors of a three-phase power system are properly connected;

FIG. 5 depicts a phasor diagram display wherein voltage probes have beeninadvertently Swapped between phases;

FIG. 6 depicts how phase connection errors may be corrected inaccordance with the invention by using a Swap operator;

FIG. 7 depicts a phasor diagram display wherein voltage probes have beeninadvertently rotated among phases;

FIG. 8-depicts how phase connection errors may be corrected inaccordance with the invention by using a Rotate operator;

FIG. 9 is a flowchart describing the steps of assigning voltage data tothe proper channel in accordance with the invention;

FIG. 10 depicts the power monitoring instrument and associated currentmeasurement pod in accordance with the invention;

FIG. 11 depicts a representation of a current pod interface inaccordance with the invention;

FIG. 12 depicts a phasor diagram display wherein an error has been madein orienting a current clamp;

FIG. 13 depicts how polarity errors made in orienting current clamps maybe corrected in accordance with the invention by using an Invertoperator;

FIG. 14 is a flowchart describing the steps of assigning current data tothe proper channel while correcting for polarity errors in accordancewith the inventions;

FIG. 15 depicts an impedance diagram of a power system as displayed inaccordance with the invention;

FIG. 16 is a flowchart describing the steps of calculating sourceimpedance in accordance with the invention;

Fig, 17 is a flowchart describing the steps of generating an impedancediagram display in accordance with the invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Fig, 1 depicts a power monitoring instrument 10 and an associatedvoltage pod 12 in accordance with the invention. The power monitoringinstrument 10 includes a voltage pod interface 14 and a signalprocessing/storage circuit 16. The power monitoring instrument 10connects to a computer system and is coupled to voltage pod 12 by afour-foot long connecting cable 20. Connecting cable 20 attaches to ahermetically sealed bayonet connector (not shown) on power monitoringinstrument 10 which seals before making electrical contact.

Voltage pod 12 is shown configured for three-phase power measurementsand includes connections to six-foot long color-coded signal cables 22,24, 26, 28, and 30 for all three phases of a multi-phase power system aswell as a neutral and ground signal respectively. In a single phaseconfiguration of voltage pod 12, two of the signal cables, 24 and 26,could be eliminated. The signal cables terminate in insulated safetyconnectors which accept various interchangeable voltage probes to allowfor connection to different conductors. For example, one type of probemay be used to attach to a threaded stud and another type may be usedfor exposed conductors. A representative voltage probe 32 is shownattached to signal cable 22. Voltage pod 12 also includes a dividercircuit, usually 1000:1, for each of the non-ground signal cables. Arepresentative divider 34 is shown in series with signal cable 22. LEDs36, 38, 40, 42, and 44 are shown for each of the signal cables.

In operation, signal processing/storage circuit 16 analyzes and storespower quality information derived from the voltages obtained via voltagepod 12. Computer system 18 is employed for further analysis and displayof power quality related information.

The use of dividers in voltage pod 12 provides safety advantages in thathigh voltage is not brought back to power monitoring instrument 10 andconnectors on the back of power monitoring instrument 10 may bemanipulated freely without fear of shock. Other safety advantages andgeneral convenience are provided by the operation of voltage podinterface 14 in conjunction with voltage pod 12.

FIG. 2 depicts a representation of voltage pod interface 14 inaccordance with the invention. Voltage pod interface 14 includes achannel mapper 100, signal detectors 102, 104, 106 and 108,corresponding to the non-ground voltage probes, and A-D converter set110. In the preferred embodiment, the functions of channel mapper 100and signal detectors 102, 104, 106, and 108 are performed by routinesexecuted by signal processing/storage circuit 16. A-D converter set 110incorporates analog-to-digital converters for digitizing the signalsarriving from the probes.

Each signal detector operates to activate the appropriate LED on voltagepod 12 upon detection of a signal. If the signal exceeds a firstpredetermined percentage of the A-D output range, the signal detectorflashes the LED. If the signal is below a second predeterminedpercentage, the LED is turned off. Thus, proper connection can bereadily verified while connecting the probes without operator attentionto power monitoring instrument 10. The time during which the operator isexposed to high voltage and the accompanying danger of shock istherefore minimized.

FIG. 3 is a flowchart describing the steps of indicating the presence ofa signal on a given voltage probe. In the preferred embodiment, thesesteps are executed by signal processing/storage circuit 16. At step 120,signal processing/storage circuit 16 checks if the instantaneous voltagelevel coming from the A-D exceeds 75% of full scale. If the signal doesin fact exceed 75% of full scale, the LED is flashed at step 130. Atstep 140, signal processing/storage circuit 16 checks if the rms voltagelevel of the signal exceeds 25% of full scale. In the preferredembodiment, signal processing/storage circuit 16 determines the rmsvoltage level by converting the digitized signal into the frequencydomain and integrating over the spectrum. If the rms voltage level ofthe signal does exceed 25% of full scale, the LED is illuminatedsteadily at step 150. If the rms voltage level of the signal does notexceed 25% of full scale, the LED is turned off at step 160.

Channel mapper 100 allows an operator to correct for inadvertentscrambling of phase connections without further handling of voltageprobes. In the preferred embodiment, keyboard commands to computersystem 18 alter the mapping between signal cables 22, 24, 26, 28 and 30and monitoring channels of signal processing/storage circuit 16.

FIG. 4 depicts a phasor diagram display 200 wherein voltage probes andphase conductors of a three-phase power system are properly connected.Phasor diagram display 200 is generated by signal processing/storagecircuit 16 or computer system and is displayed by computer system 18.Distribution panels are typically wired such that conductor phases areA, B, C, from left to right when looking at the panel.

Signal/processing storage circuit 16 has inputs for three channels 1, 2,and 3. The display shows three vectors, representing the three phasesignals respectively. Channel 1 is labeled as A, channel 2 is labeled asB, channel 3 is labeled as C, corresponding to typical distributionpanel labeling. Convention dictates that the vectors Rotate counterclockwise; first, phase A goes through 0 degrees, next phase B, andfinally phase C. The vector of each phase is displaced from its adjacentphase by 120 degrees; therefore, the vector sum of three equallybalanced phases is zero.

FIG. 5 depicts phasor diagram display 200 wherein connections for phasesB and C have been accidentally reversed. In the prior art, correction ofthis error would require the operator to physically Swap the voltageprobes for phases B and C.

FIG. 6 depicts phasor diagram display 200 wherein reversal of phaseconnections has been corrected in accordance with the invention. Theoperator may apply a Swap operator to phases B and C by appropriateinput to computer system 18. Computer system 18 then directs channelmapper 100 to couple the phase B input to channel 3 of signal/processingstorage circuit 18 and the phase C input to channel 2 ofsignal/processing storage circuit 18. The result is that phases A, B,and C appear in their desired locations on phasor diagram display 200without inconvenient operator handling of probes and the accompanyingexposure to high voltage.

FIG. 7 depicts phasor diagram display 200 for an installation whereinvoltage probe connections for phases A, B, and C have been incorrectlyconnected so that the vectors appear rotated from their correctposition. In the prior art, correction of this error would have requiredthe operator to reposition three voltage probes.

FIG. 8 depicts how the phase connection error of FIG. 7 may be correctedin accordance with the invention by using a Rotate operator. Theoperator applies a Rotate operator by appropriate input to computersystem 18. Computer system 18 then directs channel mapper 100 to couplethe A phase input to channel 2, the B phase input to channel 3, and theC phase input to channel 1. Again, the result is correction of theconnection error without further handling of voltage probes. Anyscrambling of voltage probes among phase conductors can be correctedthrough successive applications of the Rotate and Swap operators.

FIG. 9 is a flowchart describing the steps of assigning voltage data tothe proper channel in accordance with the invention, thus implementingthe functionality of depicted channel mapper 100. The flowchart is drawnin reference to the probe for phase A, but the same steps are followedfor each phase. At step 210, signal processing/storage circuit 16 readsthe voltage on the phase A probe. At step 220, signal processing/storagecircuit 16 determines whether phase A is assigned to channel 1 as itwould be if no Rotate or Swap operator had been entered as input tocomputer system 18. If phase A has been assigned to channel 1, signalprocessing/storage circuit 16 stores the phase A data on channel 1 forfurther processing at step 230. If phase A has not been assigned tochannel 1, signal processing storage circuit 16 determines whether phaseA is assigned to channel 2 as a result of the operation of Rotate and/orSwap operators at step 240. If phase A has been assigned to channel 2signal processing/storage circuit 16 stores the phase A data on channel2 for further processing at step 250. If phase A has not been assignedto channel 1 or channel 2, at step 260, signal processing storagecircuit 16 stores the phase A data on channel 3.

FIG. 10 depicts power monitoring instrument 10 and an associated currentpod 300 in accordance with the invention. Power monitoring instrument 10includes signal processing/storage circuit 16 and a current podinterface 302.

Power monitoring instrument 10 is coupled to current pod 300 by afour-foot long connection cable 304. Connection cable 304, likeconnection cable 20, attaches to a hermetically sealed bayonet connector(not shown) on power monitoring instrument 10 which seals before makingelectrical contact.

Current pod 300 is shown configured for three-phase power measurementsand includes connections to six-foot long color-coded signal cables 306,308, 310, 312, 314 for all three phases of a three-phase power system aswell as a neutral and ground signal respectively. In a single phaseconfiguration of current pod 200, two of the signal cables, 308 and 310,could be eliminated. The signal cables may couple to current clamps with5 A, 40 A, 1000 A, and 3000 A ranges. A resistor built into a connectorof each current clamp identifies the clamp and sets the correct displayrange. A burden is also built into each current clamp to prevent highvoltage shocks should the clamp become unattached from the signal cablewhile still connected to an energized conductor. A representativecurrent clamp 316 is shown attached to signal cable 306 via a resistor318. Green LEDs 320, 322, 324, and 326 mounted on current pod 300 areshown for each of the signal cables except ground signal cable 314 whichhas a red LED 328.

In operation, signal processing/storage circuit 16 analyzes and storespower quality information derived from the current measurements obtainedvia current pod 300. By integrating current measurements from currentpod 300 with voltage measurements from voltage pod 12, power monitoringinstrument 10 is able to determine source impedances and load impedancesfor a power system.

FIG. 11 depicts a representation of current pod interface 302 inaccordance with the invention. Current pod interface 302 includes apolarity correction unit 400, a channel mapper 402, signal detectors404, 406, 408, 410, 412 corresponding to the current clamps coupled tosignal cables 306, 308, 310, 312, and 314, and an A-D converter set 414.A-D converter set 414 incorporates analog-to-digital converters fordigitizing the signals arriving from the clamps. In the preferredembodiment, the functions of polarity correction unit 400, channelmapper 402, and signal detectors 404, 406, 408, 410, and 412 areperformed by software routines executed by signal processing/storagecircuit 16.

Each signal detector operates to activate the appropriate LED on currentpod 300 upon detection of a signal. For the non-ground signals, if thesignal exceeds a first predetermined percentage of the A-D output range,the signal detector flashes the LED. If the signal is below a secondpredetermined threshold, the LED turns off. The LED for the groundsignal cable turns on if more than 0.5 A of ground current is flowing.Thus, proper selection and connection of current clamps can beconveniently verified while viewing the pod without attention to powermonitoring instrument 10 or a display of computer system 18 which maynot be within easy reach. Again, the time during which the operator isexposed to high voltage and the accompanying danger of shock is thusminimized. The software routines executed by signal/processing storagecircuit 16 to implement the functions of the non-ground signal detectorsare similar to the routines described in reference to FIG. 3.

As in voltage pod interface 12, channel mapper 402 allows an operator tocorrect for inadvertent scrambling of phase connections without furtherhandling of current clamps. Phases are normally rotated and swapped inpairs such that both voltage and current are changed simultaneously. Ifthe operator must assign a current phase to a different voltage phase,current phases can be swapped and rotated independently of the initialvoltage phase assignment. However, if any current and voltage pair areseparated, all power measurements will be flagged prior to storage ordisplay.

For current measurements, another kind of connection error can be made,incorrectly orienting a current clamp on a conductor. The consequence ofthis error is that measured current will have the wrong polarity. In theprior art, this error was corrected by physically reorienting thecurrent clamp.

FIG. 12 depicts phasor diagram display 200 wherein an error has beenmade in orienting a current clamp. The current clamp for phase C hasbeen misoriented.

FIG. 13 depicts how polarity errors made in orienting current clamps maybe corrected in accordance with the invention by using an Invertoperator. The operator applies an Invert operator to channel 3, byappropriate input to computer system 18. The Invert operator is an inputto polarity correction unit 400 which responds by flipping the polarityof channel 3. Again, the adjustment is performed without returning tothe monitoring points.

FIG. 14 is a flowchart describing the steps of assigning current data tothe proper channel while correcting for polarity errors, thusimplementing the functions of polarity correction unit 400 and channelmapper 402. The flowchart is drawn in reference to the probe for phaseA, but the same steps are followed for each phase. At step 430, signalprocessing/storage circuit 16 reads the voltage on the phase A probe. Atstep 432, signal processing/storage circuit 16 determines whether phaseA is assigned to channel 1 as it would be if no Rotate or Swap operatorhad been entered as input to computer system 18. If phase A has beenassigned to channel 1, signal processing/storage circuit 16 thendetermines if an Invert operator has been applied to channel at step434. If an Invert operator has been applied to channel 1, the data fromthe phase A probe is inverted at step 436 to correct polarity. Whetheror not an Invert operator has been applied to channel 1, the phase Adata is stored on channel i for further processing at step 438.

If phase A has not been assigned to channel 1, signal processing storagecircuit 16 determines whether phase A is assigned to channel 2 as aresult of the operation of Rotate and/or Swap operators at step 440. Ifphase A has been assigned to channel 2, signal processing/storagecircuit 16 then determines whether an Invert operator has been appliedto channel 2 at step 442. If an Invert operator has been applied tochannel 2, the data from the phase A probe is inverted at step 444.Whether or not an Invert operator has been applied to channel 2, thephase A data is stored on channel 2 for further processing at step 446.

If phase A has not been assigned to channel 1 or channel 2, signalprocessing storage circuit 16 determines whether an Invert operator hasbeen applied to channel 3 at step 448. If an Invert operator has beenapplied to phase 3, the phase A data is inverted at step 450. Whether ornot an Invert operator has been applied, signal processing storagecircuit 16 stores the phase A data on channel 3 for further processingat step 452.

FIG. 15 depicts an impedance diagram 500 of a power system as displayedin accordance with the invention. A power source 502 is modeled as acombination of an ideal (zero impedance) power source 504 and a sourceimpedance (Z_(s)) 506. A load is modelled as a load impedance (Z_(L)),508. The distribution system 510 for feeding power from the power source502 to the load 508 is modeled as a series distribution impedance(Z_(DS)) 512 and a parallel distribution impedance (Z_(DP)) 514. Thediagram depicts monitoring points 516 and 518 for monitoring voltage andcurrent in the course of evaluating the depicted impedances.

FIG. 16 is a flowchart describing the steps of calculating sourceimpedance in accordance with the invention. To determine the sourceimpedance for the power source, the operator connects voltage probes andcurrent clamps coupled to power monitoring instrument 10 at monitoringpoint 516 to measure the voltage and current outputs of power source502. Unlike in prior art techniques, there is no necessity to disconnectload 508.

At step 600, signal processing/storage circuit 16 within powermonitoring instrument 10 measures rms voltage and rms current for aparticular cycle. In the preferred embodiment, the rms measurements areobtained by sampling a cycle of the voltage and current signals,digitizing the samples, transforming the samples into the frequencydomain, and then integrating over the frequency spectrum to obtain therms values.

At step 610, signal processing/storage circuit 16 calculates a loadimpedance for the given cycle by dividing the measured rms voltage byrms current. Load impedance as measured at monitoring point 516 willordinarily vary over time in response to incidental load variationscaused by equipment cycling on and off and other variations in loadcurrent requirements. At step 620, power monitoring instrument willrepeat steps 600 and 610 until two cycles with disparate load impedancesare identified. In the preferred embodiment, a 10% difference in loadimpedances is considered useful for calculating source impedance.

At step 630, signal processing/storage circuit 16 estimates a sourceimpedance by calculating the ratio of rms voltage change over rmscurrent change for the identified two cycles with disparate loadimpedances. At step 640, this estimate is stored.

At step 650, steps 600, 610, 620, 630, and 640 are repeated to generatesuccessive estimates. Statistical data is generated and maintained forthe successive estimates and when the standard deviation falls below onesigma, the mean of the estimates: is displayed by computer system atstep 660. Thus, source impedance 506 is measured without altering theinterconnections between the source 502 and load 508.

FIG. 17 is a flowchart describing the steps of generating impedancediagram display 500 in accordance with the invention. Power monitoringinstrument 10 can generate the wiring diagram of FIG. 12 for display bycomputer system 18 from measurements of voltage and current atmonitoring points 516 and 518.

The operator first couples voltage probes and current clamps tomonitoring point 516. Signal processing/storage circuit 16 monitors thevoltage and current output of power source 502 at step 700. At step 710,signal processing/storage circuit 16 evaluates the source impedance fromcycle-to-cycle variations of voltage and current output of power source502 as described in connection with FIG. 16.

The operator then couples voltage probes and a current clamp tomonitoring point 518 so that power monitoring instrument 10 can monitorthe voltage and current inputs of load 508 at step 720. At step 730,signal processing/storage circuit 16 divides a long-term mean rms inputvoltage by a long-term mean rms input current to obtain the loadimpedance.

At step 740, signal processing/storage circuit 16 estimates an apparentsource impedance as measured at the load using the techniques describedin connection with FIG. 16. This value is subtracted from the sourceimpedance value obtained in step 720 to derive a total distributionsystem impedance (Z_(D)). At step 750, the distribution system seriesimpedance (Z_(DS)) 510 is derived from the formula Z_(DS) =Z_(D) (2I_(L)-I_(s))/I_(L). At step 760, distribution system parallel impedance(Z_(DP)) 512 is derived by subtracting Z_(DS) from the total impedancefor distribution system 510. At step 770, the wiring diagram 500 of FIG.12 is displayed by computer system 18 with the calculated values for thedepicted impedance elements.

The invention has now been explained with reference to specificembodiments. Other embodiments will be apparent to those of ordinaryskill in the art upon reference to the foregoing description. Forexample, the functionality of power monitoring instrument 10 andcomputer system 18 could be incorporated into a single apparatus. It istherefore not intended that this invention be limited, except asindicated by the appended claims.

What is claimed is:
 1. A method of measuring a source impedance of analternating current source coupled to a load comprising the stepsof:detecting voltage and current outputs of the source using a voltageprobe and a current clamp; converting measured root-mean-square (rms)output voltages and rms output currents of the alternating currentsource over successive individual cycles to a sequence of load impedancemeasurements; identifying a pair of cycles having disparate loadimpedance measurements, wherein cycles of said pair have measured rmsvoltages E₁ and E₂, measured rms currents I₁ and I₂ and associated loadimpedance measurements, E₁ /I₁ and E₂ /I₂ ; and calculating an estimateof the source impedance of the alternating current source by calculatinga magnitude of (E₂ -E₁)/(I₂ -I₁).
 2. The method of claim 1 furthercomprising the steps of:identifying successive pairs of cycles havingdisparate load impedances, wherein cycles of each successive pair haverms voltages, E_(a) and E_(b), and rms currents, I_(a) and I_(b) ;calculating successive estimates of the source impedance of thealternating current source by calculating a magnitude of (E_(b)-E_(a))/(I_(b) -I_(a)) for each said successive pair of cycles havingdisparate load impedances; storing said successive estimates of thesource impedance; calculating a cumulative mean and a cumulativestandard deviation for said successive estimates of the sourceimpedance; and displaying the cumulative mean for said successiveestimates of the source impedance when said cumulative standarddeviation is below a predetermined threshold.
 3. The method of claim 1wherein said identifying step is performed by a power monitoringinstrument coupled to the voltage probe via a voltage pod comprising adivider to attenuate detected voltages.
 4. The method of claim 3 whereinsaid voltage pod further comprises an indicator for verifying a positiveconnection by the voltage probe.
 5. The method of claim 1 wherein saididentifying step is performed by a power monitoring instrument coupledto the current clamp via a current pod comprising an indicator to verifypositive connection.
 6. The method of claim 5 wherein said powermonitoring instrument comprises a polarity reversing device to correctcurrent clamp orientation errors.